Method and apparatus for control of mobility-based ion species identification

ABSTRACT

System for control of ion species behavior in a time-varying filter field of an ion mobility-based spectrometer to improve species identification, based on control of electrical and environmental aspects of sample analysis.

RELATED APPLICATIONS

This patent application is a continuation of U.S. patent applicationSer. No. 10/462,206 which is a continuation-in-part of U.S. patentapplication Ser. No. 10/321,822 filed Dec. 16, 2002, acontinuation-in-part of U.S. patent application Ser. No. 10/123,030filed Apr. 12, 2002, and a continuation-in-part of U.S. patentapplication Ser. No. 10/187,464 filed Jun. 28, 2002, and claims thebenefit of U.S. Provisional Application No. 60/389,400 filed Jun. 15,2002, claims the benefit of U.S. Provisional Application No. 60/398,616filed Jul. 25, 2002, claims the benefit of U.S. Provisional ApplicationNo. 60/418,671 filed Oct. 15, 2002, claims the benefit of U.S.Provisional Application No. 60/453,287 filed Mar. 10, 2003, and claimsthe benefit of U.S. Provisional Application No. 60/468,306 filed May 6,2003. The entire teachings of the above-identified applications areincorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to methods and apparatus for detection andidentification of substances in general, and more particularly tomethods and apparatus for analysis of ions by ion mobility.

BACKGROUND OF THE INVENTION

There are many situations where it is desired to identify chemicalcompounds in a sample. Such samples may be taken directly from theenvironment or they may be provided by front end specialized devices toseparate or prepare compounds before analysis. Furthermore, recentevents have seen members of the general public exposed to dangerouschemicals in situations where previously no thought was given to suchexposure. There exists, therefore, a demand for low cost, accurate, easyto deploy and use, reliable devices capable of identifying the chemicalcontent of a sample.

One class of known chemical analysis instruments is referred to as massspectrometers. Mass spectrometers are generally recognized as being themost accurate type of detectors for compound identification, given thatthey can generate a fingerprint pattern for even fragment ions. However,mass spectrometers are quite expensive and large and are relativelydifficult to deploy in the field. Mass spectrometers also suffer fromother shortcomings such as the need to operate at low pressures,resulting in complex support systems. These systems also require ahighly trained user to tend to operations and interpret results.

Another class of known chemical analysis instruments enable used ofatmospheric-pressure chemical ionization. Ion analysis is based on therecognition that ion species have different ion mobility characteristicsunder different electric field conditions at elevated pressureconditions including atmospheric pressure. Practices of the conceptinclude time-of-flight Ion Mobility Spectrometry (IMS) and differentialmobility spectrometry (DMS), the latter also sometimes referred to asfield asymmetric ion mobility spectrometry (FAIMS). These systems enablechemical species identification at atmospheric pressure, preferablybased on dry and clean gas samples.

In a conventional time-of-flight IMS device (sometimes referred to asTOF-IMS), a propelling DC field gradient and a counter gas flow are setand an ionized sample is released into the field which flows to acollector electrode. Ion species are identified based on the DC fieldstrength and time of flight of the ions to the collector. The electricfield is weak where ion mobility is constant.

DMS systems identify ion species by mobility behavior in a highasymmetric RF field, where ions flow in a carrier gas and are shifted intheir path by an electric field. The conventional DMS operates with at aselected RF field at Vmax and species detections are correlated with apre-set, or scanned, DC compensation voltage (Vc). Species areidentified based upon correlation of Vmax and Vc with historical detectdata. It is well-known that for a given ion species in a sample, as theamplitude of the asymmetric RF voltage (at Vmax) changes, the amplitudeof the DC compensation voltage (Vc) required for passage of that speciesthrough the filter field will also change. The amount of compensationdepends upon species characteristics.

A typical DMS device includes a pair of opposed filter electrodesdefining an analytical gap between them in a flow path (also known as adrift tube). Ions flow into the analytical gap. An asymmetric RF field(sometimes referred to as a filter field, a dispersion field or aseparation field) is generated between the electrodes transverse to thecarrier gas/ ion flow in the gap. Field strength, E, varies as theapplied RF voltage (sometimes referred to as dispersion or separationvoltage, or Vrf) and size of the gap between the electrodes. Suchsystems operate at atmospheric pressure.

Ions are displaced transversely by the RF field, with a given speciesbeing displaced a characteristic amount toward the electrodes per cycle.DC compensation (Vc) is applied to the electrodes along with Vrf tocompensate the displacement of a particular species. Now the appliedcompensation will offset transverse displacement generated by theapplied Vrf for that particular ion species. The result is zero ornear-zero net transverse displacement of that species, which enablesthat species to pass through the filter for detection. All other ionsundergo a net displacement toward the filter electrodes and willeventually undergo collisional neutralization on one of the electrodes.

If the compensation voltage is scanned for a given RF field, a completespectrum of ion species in the sample can be produced. The recordedimage of this spectral scan is sometimes referred to as a “mobilityscan”, as an “ionogram”, or as “DMS spectra”. The time required tocomplete a scan is system dependent. Relatively speaking, a prior artIMS scan might take on the order of a second to complete while and aprior art DMS might take on the order of 10 seconds to complete.

DMS operates based on the fact that an ion species will have anidentifying property of high and low field mobility in the analytical RFfield. Thus DMS detects differences in an ion's mobility between highand low field conditions and classifies the ions according to thesedifferences. These differences reflect ion properties such as charge,size, and mass as well as the collision frequency and energy obtained byions between collisions and therefore enable identification of ions byspecies.

Illustrative examples of mobility scans based on the output from a DMSdevice are shown in FIG. 1A and FIG. 1B. As shown in FIG. 1A, a singlecompound, acetone, was submitted to the DMS analyzer. The illustratedplot is typical of the observed response of the DMS device, withdetected acetone ions in this example forming a peak intensity at acompensation voltage of about −1.5 volts. This is useful information,such that future detections of a peak at this compensation in thisdevice is indicative of detection of acetone.

In FIG. 1B, the analyzed sample consisted of acetone and an isomer ofxylene (o-xylene). The acetone peak appears at about −2.5 volts whileo-xylene appears at about −4 volts. Data representing these detectionpeaks can be compared against stored data for known compounds for thisdevice and the applied RF field and compensation, and identification ismade based upon a data match. FIG. 1B demonstrates unique detectionpeaks according to ion mobility characteristics for different ionspecies in the sample under test, i.e., o-xylene and acetone.

Various chemical species in a sample can be identified according to theconventional DMS process. However, accurate identification of severalspecies in a sample whose detection spectra overlap is difficult. Thisis in part due to the fact that DMS detection peaks are relatively broadcompared to a mass spectrometer, so overlap is more likely than with amass spectrometer. In fact, where several ion species exhibit similarbehavior in the DMS filter field their associated DC compensation willbe very close, and so their detection spectra (detection peaks) willpresent as overlapped.

This “overlap” of detection peaks interferes with speciesidentification. But discrimination between overlapping spectra is noteasily achieved and similar species are not so easily separated.

Furthermore, false negative detections are dangerous when dangerouscompounds are at issue, while false positives can reduce trust in adetection system. Therefore improved spectrometer performance is animportant goal of the present invention.

It is therefore an object of the present invention to provide a fast andsimple system, whether method or apparatus, capable of a high degree ofspecies discrimination and accurate species identification for chemicalanalysis.

SUMMARY OF THE INVENTION

A system of the invention, whether as method or apparatus, provides forcontrol of ion species behavior in a time-varying filter field of an ionmobility-based spectrometer. In practice of the invention, the filterfield has both electrical and environmental aspects that are manipulatedto improve system performance and to fine tune sample analysis.

One illustrative system of the present invention has several aspects,including: detecting and provisionally identifying at least one ionspecies, typically one out of several ion species with overlappingspectra, at a first set of filter operating conditions; selectivelychanging these operating conditions based upon the first detection andpredicting the effect of such change upon the behavior of suchprovisionally identified species; and then confirming the provisionalspecies identification based upon detection of the predicted behavior.Furthermore, additional detections can be made to further assureaccuracy of detection. It is noted that after species are separated,they are passed for downstream use or further processing, such as forspecies detection and identification.

In a further embodiment of the present invention, a sample is analyzedin a DMS filter of the invention at a first set of filter operatingconditions and one or several ion species that pass through the filterare detected. The first set of operating conditions is selected basedupon interest in monitoring for a chosen species or range of species orbased upon interest in generating a spectral scan for a chemical sample.Next, especially in the case where presence of overlapping detectionpeaks is suspected, a provisional prediction of the identity of at leastone detected ion species is made based on knowledge of the parameters ofthe first set of operating conditions and by reference to a lookup tableof behavior data that includes such species.

This process continues wherein the parameters of a second set ofoperating conditions is selected according to their expected impact uponthe expected travel behavior in the filter of the provisionallyidentified ion species, again by reference to a lookup table of relevantspecies behavior data. A second detection is made, premised on causingand detecting the predicted behavior of the provisionally identifiedspecies at the second set of operating conditions. Under suchcircumstances, an affirmative detection of such predicted behaviorenables confirmation of the first provisional identification of thedetected species. This confirmation increases the reliability of thespecies identification process.

The second set of operating conditions is selected based on knowledge ofthe first set of operating conditions and with the intention ofconfirming the first detection rather than merely making an independentsecond detection. In practical effect, the second set of operatingconditions is selected to cause differential shifting of spectra, and insome cases to eliminate or reduce the spectral overlap when the seconddetection is made and spectra are evaluated.

Thus, it will now be understood that it is the combination of the firstand second detection that enables a high degree of reliability inspecies identification made according to the invention. While the firstdetection and species identification is provisional, once a confirmingdetection of the predicted behavior of the provisionally identifiedspecies at the second set of operating conditions is answered in theaffirmative, then the provisional identification of species is reliedupon as accurate. Meanwhile, if the confirming detection answers in thenegative, then a redetection under changed operating conditions iscalled for.

However, the invention also contemplates an alternative identificationprocess in which confirmation is based upon absence of detection of theprovisionally identified ion species at the second set of operatingconditions (i.e., if no detection, then it must be x; or, if adetection, then it must y), again based upon stored knowledge of speciesbehavior under known operating conditions.

It will now be understood that the confirming second set of operatingconditions is selected based upon knowledge of characteristic behaviorof the predicted detected ion species at that second set of operatingconditions. The particular parametric changes to be made are dictated bywhat is known about the behavior of the provisionally identified speciesin the DMS field. These parametric differences must cause predictableand characteristic changes in the travel behavior of the provisionallyidentified ion species. Reference to a lookup table of associatedbehavior data, or to artificial intelligence that utilizes ion behaviorknowledge, can be used to set the second set of operating conditions.

Both the first and the second set of operating conditions are defined interms of RF field, RF waveform characteristics, applied fieldcompensation, and environmental factors (e.g., content and flow).However, we have found that to assure a high level of accurate speciesprediction, the changes in the parameters of the first set of operatingconditions include changes to the RF waveform characteristics and/orchanges to the environmental aspects of the operating conditions. Thesechanges are aside from possible changes to the RF field and DCcompensation. However it is further noted that field strength changesalone are not reliable or sufficient in these overlap situations.

In practice of the present invention, we improve DMS species detectionand identification by improving species separation. Thus, in practice ofthe present invention, an ion species is identified by making a firstdetection at a first set of filter operating conditions and thenfollowed by a second detection at a second related set of selectedoperating conditions. This process includes noting of operatingconditions and then noting changes in ion behavior after adjusting theseconditions.

It will thus be appreciated that species identification is based onobtaining related data points for a detected species. Creating and usingmultiple data points increases accuracy and wisely selecting these datapoints both increases accuracy and reduces the data processing workload.The method of making, generating and using such data points is part ofthe present invention.

In one process of the invention, we make a first ion species detectionat a first set of operating conditions. This first set of conditions isexpressed as a first parameter set of mobility-influencing variables,i.e., RF frequency, field strength, duty cycle, compensation level,pressure, humidity, flow rate, gas composition, etc. We provisionallyidentify the detected species based on historical data. We thenestablish a second parameter set of field variables to make a second (orconfirming) detection at a second (or confirming) set of operatingconditions. This second parameter set of variables is selected in viewof the first detection, and this detection of expected detection dataconfirms accuracy of the provisional identification. In a furtherpractice of the invention, a third parameter set of field variables isused to make a third detection at a third set of operating conditions tofurther confirm species identification.

In a special embodiment of the invention, a device is dedicated todetection of a prescribed analyte and detection is made at a prescribedset of operating conditions. Then positive and negative detection modedata, and/or data from detection at a second set of prescribed operatingconditions, is used for species identification.

Thus it will be appreciated that in practice of the present invention weimprove species identification by improving separation between analytes.We do this by controlling mobility-impacting aspects of the filterfield, which includes a process we generally refer to as “waveformcontrol”. We decide which parameters of the field to adjust based onknown species behavior. We set the spectrometer to detect a givenspecies or class of species and then refine the filter field anddetection process to improve species separation. Adjustments to thefilter field are selectively made in terms of field, DC compensation,frequency, duty cycle, and/or asymmetry and in terms of pressure, flowrate, gas composition, moisture, ionization process, and/or presence andlevel of doping. The result is improved species separation and improvedspecies identification.

It will be appreciated that we can optimize ion species analysis inpractice of the illustrative apparatus discussed below by making any oneof several adjustments to the filter operating conditions and makingmultiple detections. Specifically, we identify and control electricaland environmental aspects of the spectrometer filter field. We makeadjustments to these electrical and environmental aspects as if theywere “knobs” to improve species analysis. We have identifiedspecies-specific adjustments and therefore we teach their use as aids inspecies discrimination. The result is improved specificity andsensitivity in atmospheric pressure chemical species analysis.

Generally speaking, we divide adjustment to the filter field conditionsinto two categories: electrical and environmental. These adjustments aremade for specific purposes to achieve prescribed results for detectedconditions and are made based upon knowledge of the affect theseparametric adjustments will have on system performance and analytebehavior. With such assurance and the fact that we identify an ionspecies with multiple data points, our species identifications arehighly accurate with minimized false detections.

It is known that in DMS prior art, a particular ion species can bedetected by setting certain combinations of RF characteristics andstrength and DC compensation for the ion filter field. If the values ofthe RF and DC are fixed, then the system is dedicated to detection of aparticular ion species of interest, but if the DC compensation voltageis scanned through a range of voltages, then a complete mobility scancan be generated for the sample under test. This scan is based on theconventional practice of establishing an RF filter field at a givenfield strength and given frequency and then scanning the DCcompensation. Different species are compensated at different DCcompensation levels. Therefore theoretically a scan of DC compensationwill provide a scan of the chemical sample under test.

It will be appreciated that mobility of ion species in the filter fieldmay change responsively and characteristically as parameters of thefilter operating conditions are changed, and that these responses aredifferent from just scanning the DC compensation. Thus, in order toimprove species discrimination, especially in complex samples, we havefound that in addition to or combined with DC compensation, we can setand/or scan field parameters, which can be set at a fixed value or canbe scanned through a range of values, to affect ions in the field and totune the field to pass a particular limited set of ions or ion species.This scanning may include stepping or sweeping through a range ofvalues. The particular parameters are selected based on predicted impacton behavior of species of interest.

We can mix various combinations of these adjustments in aspecies-specific manner to improve species discrimination. As a result,we provide better separated ion species to a detector for improve sampleanalysis. This detector may be on-board or otherwise. In one embodimentthe present invention provides a mobility-based pre-filter for a massspectrometer.

It will be further appreciated that the present invention does not needto follow the conventional wisdom of IMS and DMS of analyzing thechemical sample at or about atmospheric pressure and at reduced or zerohumidity. In fact, we come to recognize that pressure and humidity areparameters that can be favorably adjusted and quite unexpectedly thatbenefits can be derived from operation of an atmospheric pressure ionmobility detection system at other than atmospheric pressure and/or atelevated humidity to achieve improved ion species separation.

Therefore, in several embodiments of the invention, we provide andregulate pressure and/or humidity to favorably and differentially affectand control ion species separation in the electric field. In suchembodiments, the operating pressure need not be at the conventionalatmospheric level, and the humidity need not be at the conventionaltrace level, wherein we can choose to optimize these parameters tocompensate ion mobility and to favorably control the analytical processand consequent species identification.

This invention has practical applications. For example, we canaccurately separate, detect and identify chemical species, even thoughit may be a difficult chemical to isolate in an air sample. In onepractice of the invention, we select an RF intensity and adjust pressureand humidity to desired values, based on known species data, and then weperform a mobility scan by scanning the DC compensation voltage todetect acetone and sulfur hexafluoride (SF6) in a sample containing air,acetone and SF6. The air, acetone and SF6 are easily separated, detectedand identified in this illustrative practice of the invention.

It will now be appreciated that the concept of applying compensation tothe analytical filter field is broader than the conventional concept ofvarying the DC compensation voltage. In short, we have recognized thatthere are numerous “controls” or “knobs” which may be adjusted in amanner that predictably affects ion mobility for the purpose ofcompensating (or tuning) the electric field to pass ion species to thedetector. The result is improved specificity in species discriminationand detection, especially in complex samples. The benefit is increasedaccuracy in species identification with reduced false positives andreduced false negatives.

In a method of the invention, we control and adjust operating conditionsby several techniques. For example, we can adjust the electric fieldfrequency in a DMS system, which affects the ‘selectivity’ (width) ofthe scanned peaks in the detector output or filter. WE can alsoselections, such as light versus heavy ions for separation.

The process can be implemented by changing the value of a fixedoperating frequency or by dynamic frequency modulation where a range offrequencies could be scanned, for example. As well, the waveform (i.e.,square, triangular, sinusoidal, ramp, etc.) may be adjusted, whereinpulse shape is used to affect response of the ion in the field in aknown manner. This control may be augmented by adjusting the analyticalgap environmental parameters (such as by changing the pressure and/orconcentration of water, other polar molecules, or other dopants) topositively affect response of ion species in the field.

In yet another embodiment of the present invention, a DMS deviceoperates simultaneously in both positive ion detection mode (“positivemode” or “positive ion mode”) and negative ion detection mode (“negativemode” or “negative ion mode”) for a more complete real-time sampleanalysis. Therefore another practice of the invention detects andseparates multiple species simultaneously based on both ion mobility andion polarity.

We broadly define doping as the process of adding an analyte to a sampleflow for the purpose of affecting ion species behavior. We can use thisdoping to assist in identifying analytes of interest. We define severalforms of doping.

Doping may include the step of use of a dopant additive to improveionization efficiency. Doping may include the step of addition of ananalyte in the ionization process whose ionization releases freeelectrons which enables formation of negative ions for species with highelectron affinity. Doping may include the step of addition of an analytethat affects species behavior and causes peak shift.

We can combine data from these doping-assisted detections withdetections made without the benefit of dopant. The characteristics of agiven chemical sample will dictate its ionization in these conditions,i.e., a signature. Thus it will be appreciated that in an embodiment ofthe invention multiple detect data are compared against stored detectiondata so as to be able to make positive and reliable species signatureidentification.

In an illustrative practice of the invention, we note detection peakcharacteristics (ion polarity, number of peaks, peak location,intensity, width, etc.) at a first set of operating conditions (notingionization source, dopant level if any, and other electric andenvironmental field parameters, including Vmax, Vmin, Vc, RF frequency,duty cycle, etc.). We then change at least one mobility-effectingparameter in the operating conditions (such as adjusting dopant level,Vmax (and/or the ratio of Vmax/Vmin), Vc, RF frequency, waveshape and/orduty cycle) and note changes in peak characteristics (such as location,intensity, width, etc.) at the second set of operating conditions. Thiscollected data is compared to a lookup table of detection data for knowncompounds in such conditions. Upon data match, a species identificationis made with a high degree of reliability. Furthermore, in preferredpractice of the invention, we make the second detection at a second setof operating conditions that is selected guided by knowledge of thefirst set of operating conditions and the first detection results, withat least one change being in made in parameters of the operatingconditions, preferably one that includes other than merely changingfield strength and adjusting DC compensation. In one example, the seconddetection amounts to a measurement of peak shift associated with thechange in operating conditions.

In another illustrative embodiment of the invention, a DMS method foridentifying chemical species in a sample includes several steps. Thesystem provides a DMS filter field which is adjustable to a plurality ofDMS filter operating conditions. The DMS filter operating conditions arecharacterized as influencing mobility behavior of ions in the filter.Behavior of the ionized sample is analyzed in the filter at a first setof operating conditions, with the sample including at least one ionspecies and the analysis being based upon aspects of mobility behaviorof the at least one ion species in the first set of operatingconditions. This is followed by detecting a spectral peak associatedwith the at least one ion species and the first set of operatingconditions. The next step includes provisionally identifying the atleast one ion species based upon that species detection and theassociated operating condition parameters. Based upon the provisionalidentification, a change is made to parameter(s) of the operatingconditions at least in terms of waveform characteristics, RF frequency,duty cycle, gas composition, pressure, presence of dopant, or flow rate,and predicting the effect of such change upon the provisionallyidentified at least one ion species measured in terms of change in atleast one characteristic of the spectral peak. A change in the spectralis expected. A detection is made of the spectral peak associated withthe at least one ion species at the second set of operating conditionsto confirm the predicted change. Based on the first detection and theconfirmation, which verifies the provisional identification of the atleast one ion species, an announcement is made identifying the at leastone ion species. This identification is made with a high degree ofreliability.

The present invention may be practiced in ion mobility-based systems,including IMS and DMS, and may have various cylindrical, planar, radialand other structural configurations. It will be further appreciated thatmethods of the invention include one or all of the following actions:separation, detection and/or identification of ion species according toaspects and/or changes in mobility behavior in a controlled filterfield. These terms may be generally referred to as ion species“analysis”.

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains at least one drawing executed incolor. Copies of this patent or patent application publication withcolor drawing(s) will be provided by the Office upon request and paymentof the necessary fee.

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of illustrative and preferred embodiments of the invention,as illustrated in the accompanying drawings in which like referencecharacters refer to the same parts throughout the different views. Thedrawings are not necessarily to scale, emphasis instead being placedupon illustrating the principles of the invention, wherein:

FIG. 1A and FIG. 1B are a prior art mobility scans plotting detectionintensity versus compensation voltage for a given field strength in afield asymmetric ion mobility spectrometer, for acetone alone (1A) andfor a combination of o-xylene and acetone (1B).

FIG. 2A is a schematic of a differential ion mobility spectrometer inaccordance with an embodiment of the present invention.

FIG. 2B shows a curved filter electrode embodiment of the presentinvention.

FIG. 2C is a multi-channel differential ion mobility spectrometer inaccordance with an embodiment of the present invention.

FIG. 3 shows positive and negative mode detections for methyl salycilatein an RF field operating at Vmax of 1100 v, in practice of theinvention.

FIG. 4 shows the effect of frequency on positive mode backgroundspectra, in practice of the invention.

FIG. 5 shows the effect of frequency on negative mode backgroundspectra, in practice of the invention.

FIG. 6 shows the effect of electric field strength on negative andpositive RIP peak parameters for two frequencies, in practice of theinvention.

FIGS. 7-8 show the effect of frequency and of electric field strength onpositive (A) and negative (B) acetone ion peaks, in practice of theinvention.

FIGS. 9-10 show the effect of frequency and of electric field strengthon positive (A) and negative (B) benzene ion peaks, in practice of theinvention.

FIGS. 11-12 show the effect of frequency and of electric field strengthon positive (A) and negative (B) toluene ion peaks, in practice of theinvention.

FIG. 13A-B show flyback and squarewave waveforms, in practice of theinvention.

FIG. 14A-B show detection spectra corresponding to the waveforms of FIG.13A-B, in practice of the invention.

FIG. 15 A, shows the effect of frequency of the RF voltage upon RIP,toluene and SF6 ion peaks, in practice of the invention.

FIG. 15B shows the effect of duty cycle of the RF voltage upon SF6 ionpeaks, in practice of the invention.

FIG. 15C shows the effect of duty cycle of the RF voltage upon RIP andtoluene ion peaks, in practice of the invention.

FIG. 16A-B show negative (A) and positive (B) spectra for differentconcentrations of SF6, in practice of the invention.

FIG. 17 omit.

FIG. 18 shows the effect of doping on heptanone ions, in practice of theinvention.

FIG. 19 shows the effect of doping on butanone ions, in practice of theinvention.

FIG. 20 shows the effect of doping on DMMP ions, in practice of theinvention.

FIG. 21 shows dopant effect upon explosives detection (DNT), in practiceof the invention.

FIG. 22A-E shows undoped detection of explosive compounds.

FIG. 23A-E shows doped detection of explosive compounds, using MC dopantin practice of the invention.

FIG. 24A shows a composite of the detections of FIG. 22.

FIG. 24B shows a composite of the detections of FIG. 23, in practice ofthe invention.

FIG. 25A-E shows effect of polar doping on DNT with differentconcentrations of water, in practice of the invention.

FIG. 26A-C show peak positions for different concentrations of water (A)and DNT/water (B) and a plot of DNT/water peak versus waterconcentration (C).

FIG. 27A-C show detection peaks for DMMP, DEMP and DEIP at moisturelevel of 6 ppm (A) and 95000 ppm (B) and a plot of peak position versusmoisture (C), in practice of the invention.

FIG. 28 shows a dopant control apparatus, in practice of the invention.

FIG. 29A-B shows effect of pressure on negative (A) and positive (B)mode background spectra, in practice of the invention.

FIG. 30A-B shows effect of pressure on negative (A) and positive (B)background ion peak parameters, in practice of the invention.

FIG. 31A-B shows adjustment of RF voltage for changes in pressure forpositive (A) and negative (B) background spectra, in practice of theinvention.

FIG. 32A-B shows quantified effect of electric field compensation forpressure decrease for positive (A) and negative (B) background spectra,in practice of the invention.

FIG. 33A-B shows the effect of pressure on negative (A) and positive (B)TBM spectra, in practice of the invention.

FIG. 34 A and B shows the effect of pressure on negative (A) andpositive (B) TBM ion peak parameters, in practice of the invention.

FIG. 35A-B shows effect of reduced pressure on negative (A) and positive(B) SF6 spectra, in practice of the invention.

FIGS. 36-37 show improved flow control apparatus of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Illustrative DMS embodiments of the present invention are shown in FIG.2A, 2B, and 2C. In the embodiment of FIG. 2A, apparatus 10 has an inlet12 that accommodates the flow of a carrier gas G carrying sample S intothe device and then along flow channel 13. The sample is drawn from theenvironment or received from a front end device, such as a gaschromatograph, and flows from inlet 12 to ionization region 14 along theflow path.

Compounds in the sample are ionized by an ionization source 16 as thesample flows through ionization region 14, creating a set of ionizedmolecules 17+, 17−, accompanied by some neutral molecules 17 n ofvarious chemical species. Ionized monomers and/or dimers are createdduring such ionization. Also clusters of ions may be created when amonomer combines with water molecules or other background molecules, inan ionized combination.

The ions are carried by a gas stream (sometimes referred to as a carriergas) through stages of the system (e.g., into filter 24 and to detector32), such as taught in U.S. Pat. No. 6,495,823, incorporated herein byreference. Alternatively, the sample may be conveyed via electric field,with or without carrier gas, as taught in U.S. Pat. No. 6,512,224,incorporated herein by reference.

In the embodiment of FIG. 2A, carrier gas G carries the ions intoanalytical gap 18 between filter electrodes 20, 22 of ion filter 24. Acompensated asymmetric RF filter field F is developed between the ionfilter electrodes in the analytical gap between the electrodes (e.g.,0.5 mm). The strength of the field varies according to the applied RFvoltage (Vrf) in the gap.

In the embodiment of FIG. 2A, a detector 32 is on-board system 10 andtakes the form of at least one electrode, and preferably includes aplurality of electrodes, such as opposed electrodes 28 and 30,associated with the flow path downstream of filter 24. The detector maybe of various kinds, whether as complex as a mass spectrometer or assimple as opposed electrodes as shown in FIG. 2A. As well, in anotherembodiment, the detector is ccd-based which provides improved iondetection sensitivity. In yet another embodiment, the invention improvesspecies separation as a front end to other processes, and does notrequire a detector.

Control unit 40 preferably performs a number of important actions inaccordance with the present invention, and may incorporate variousdevices or functions for this purpose. These may include RF voltagegenerator 42, compensation voltage generator 44, a microprocessor unit(MPU) 46, memory 47, an analog-to-digital converter 48, and display 49.

Microprocessor 46 provides digital control signals to the RF voltagegenerator 42 and optional compensation voltage generator 44 to generatethe desired compensated drive voltages for filter 24. These devices mayalso include digital-to-analog converters and the like, although notshown in detail.

In the embodiment of FIG. 2A, control unit 40 biases and monitors theelectrodes 28, 30 of detector 32. Microprocessor 46 correlates appliedcompensation and RF voltages with observed responses at detector 32, viaanalog-to-digital converters 48. By comparing an observed response of,for example, peak detection intensity for a particular ion species atleast two data points selected according to principles of the invention,the microprocessor 46 can identify particular compounds by comparisonwith a library of data stored in its memory 47. The result of thecomparison may then be announced at an appropriate output device such asa display 49, or may be provided by electrical signals through aninterface 50 to other computer equipment.

Apparatus of the invention are very stable and test results arerepeatable. Therefore, in a preferred practice of the invention, we usethe history table (lookup table) for species of ions that have beendetected as correlated with compensation, RF and other field conditions,which enables use of the device for identification of detectedchemicals. It is also within the scope of the invention to calibrate thesystem using the reactant ion peak (RIP) or a dopant peak, for example,among other techniques.

It will be appreciated that ions are separated based on differentialmobility in the filter field F in the analytical gap 18 according toexisting field conditions. Field F can be held at a fixed value, whereinthe system is dedicated to detection of a particular ion species at asingle data point, or the field conditions can be varied for generationof a plurality of data points. As well, a particular field parameter canbe scanned to generate a mobility scan, wherein field conditions are setto a particular value except for at least one mobility-affectingparameter that is swept through a range so as to generate a mobilityspectrum for the sample under test. This is performed under directionand control of control unit 40.

The embodiment of FIG. 2A has a flow path with generally flat contour.This is shown by way of illustration and not limitation. The presentinvention is not limited to flat plate configurations and may bepracticed in other configurations, including, among others, radial,coaxial and cylindrical DMS devices. For example, an illustrative DMShaving curved flow path 13′ between curved filter electrodes 20′, 22′(which may include curved plate electrodes or concentric cylindricalelectrodes, among others) is shown in FIG. 2B, and an illustrativemulti-channel embodiment 11 is shown in FIG. 2C (discussed belowregarding use of dopants).

Simultaneous Analysis Modes

Another feature of the apparatus of FIG. 2A is that both positive andnegative ion species can be analyzed simultaneously. Normally, a singlechannel APCI spectrometer detects ions of either positive or negativeion species in any one operating period, but not both simultaneously. Ifa single sample supplies both positive and negative ions, then multipledetections must be run seriatim for a complete analysis in a singlechannel system. Obviously multiple systems can be run simultaneously butthis is both expensive and impractical.

Embodiments of the present invention overcome this limitation withvarious strategies based on ion flow design. For example, spectrometer10 of FIG. 2A can generate, filter and detect both positive and negativeions simultaneously. These positive and negative ions can be related tothe same or different chemicals in the sample. This simultaneousfunctionality is set forth in copending U.S. patent application Ser. No.10/187464, filed Jun. 28, 2002 (Attorney Docket No. M070), which is acontinuation-in-part of U.S. patent application Ser. No. 09/896,536,filed Jun. 30, 2001, entitled “Apparatus For Simultaneous IdentificationOf Multiple Chemical Compounds,” both of which are incorporated hereinby reference.

We refer to detection of ions as detection modes: i.e., as positivedetection mode (or positive ion mode, or, simply, positive mode), whenpositive ions pass through the filter and are attracted and detected bya negatively biased detector electrode, and negative detection mode (ornegative ion mode, or, simply, negative mode), when negative ions passthrough the filter and are attracted and detected by a positively biaseddetector electrode. Having both electrodes 28, 30 in detector 32 enablessimultaneous detection of positive and negative ion speciessimultaneously passed by filter 24.

More particularly, as shown in FIG. 2A, positive and negative ions 17+,17− are generated in ionization region 14 and are introduced into filter24 (within analytical gap 18). If these ions have different mobilityunder a given set of compensated RF field conditions, then just the17+or 17− ions will be passed by the filter while all other ions will beneutralized, as in conventional DMS. This passage defines the passedions as a single-polarity ion species.

Detection will proceed at a detector electrode. In this embodiment, ifelectrode 28 is positively biased, then it will attract ions 17− whichwill be detected upon their contact with the electrode. If electrode 28is negatively biased, then it attracts ions 17+ which will be detectedupon their contact with the electrode. Electrode 30 may be used in alike manner. The charge deposits at electrodes 28 or 30 are amplified byrespective amplifiers 36 and 38, to provide detection data for use incontrol unit 40 for identification of the detected ion species. We callthis a single mode detection.

However embodiments of the present invention are also capable of dualmode detections, having dual detector electrodes. In fact, whenperforming a mobility scan on a sample we can detect negative andpositive ions passing through the filter within a single mobility scan.We refer to this process as “dual” or “simultaneous” detection becauseboth positive and negative ions can be detected in one scan. The twodetection modes include: positive detection mode where peak intensityassociated with detection of positive ions passing through the filter isdetected and negative detection mode where peak intensity associatedwith detection of negative ions passing through the filter is detected;these detections may be simultaneously displayed. In practice, it is theoutput of each detector electrode that is monitored to generate thepositive and negative mode mobility peaks (i.e., spectra).

Furthermore, this simultaneous detection within one scan also includesthe case where positive and negative ions are of such similar mobilityunder the same set of compensated field conditions that both are passedsimultaneously through filter 24 as one “mobility species”.Nevertheless, these ions do not interfere and their peaks do not overlapin the practice present invention because an additional separation stepoccurs at the detector. This additional separation step takes the formof having biased detector electrodes that separate the ions by polarity.

Thus preferred embodiments of the invention incorporate a dual mode,simultaneous detection capability. The embodiment of FIG. 2A isconfigured to distinguish between and to detect dual ion modes. Forexample, if electrode 28 is positively biased, then it attracts ions 17−and repels ions 17+ toward electrode 30. If electrode 30 is negativelybiased, then it attracts ions 17+ while repelling ions 17− towardelectrode 28. Thus this final stage of separation separates ions bydepositing their charges on the appropriately biased detector electrodes28 or 30, which may occur simultaneously. These charge deposits atelectrodes 28 and 30 are amplified by respective amplifiers 36 and 38,which may be operated simultaneously, to provide detection data for usein control unit 40 for identification of both modes of the detected ionspecies, simultaneously from a single compensation scan.

It is further noted that some single chemical species may form bothpositive and negative ions. For example a complex molecule may fragmentunder test conditions. The negative mode spectra may be the same ordifferent from the positive mode spectra depending upon mobility, onpositive and negative ions and fragmentation. Thus we can use the datafrom both modes in a single detection scan to better identify thetotality of detected chemical(s). This is beneficial even where speciesions only are detected in one or the other mode, since the fact ofpresence and absence of data assists in species specificidentifications.

It will therefore be appreciated that detection of both polarity modesin a sample analysis yields additional information in ion detection andidentification. Furthermore, simultaneous detection makes this processfaster and simpler than running multiple detections. This increased datacan result in reduced false positives, leading to a higher level ofconfidence in compound identification.

As an example, in FIG. 3, we show positive and negative mode detectionsfor methyl salycilate in an RF field operating at Vmax of 1100 v.Ionization of methyl salycilate produces negative and positive ions, butconventional spectrometers on detect only one or the other mode at onetime. In the present invention, both modes can be detected and displayedsimultaneously, enabling faster an more reliable detection based onthese multiple data detections. Detection of either mode may be thebasis of identification of the ionized methyl salycilate based on resortto a lookup table (library) that includes relevant detection data. But amore reliable identification is made by comparing the detection data forboth positive and negative mode detections. In a preferred practice ofthe invention, this occurs simultaneously. This dual mode aspect of theinvention enables improved species identification based on multipledetection data.

Enhanced Species Discrimination By Control of Waveform Parameters ofFilter Operating Conditions:

In practice of the present invention, we can optimize ion speciesanalysis by making any one of several adjustments to operatingconditions and making multiple detections. Making of these adjustmentsmay be described in operational terms as making adjustment to “knobs”45, FIG. 2A, associated with such properties. Nevertheless, we dividethese parametric adjustments into two categories: electrical andenvironmental.

Electrical adjustments include adjustment to waveform characteristicssuch as field strength, DC compensation, frequency, duty cycle, and/orasymmetry, for example. Environmental adjustments include adjustments topressure, flow rate and gas composition, including use of additives ordopants that enhance ionization efficiency (such as for UV ionization)or to produce free electrons for production of negative spectra, andalso includes use of dopants for peak shifting.

These system adjustments are made for specific purposes to achieveprescribed results, based upon knowledge of the affect these parametricadjustments will have on system performance and analyte behavior. Withsuch knowledge and the fact that we identify an ion species withmultiple data points, our species identifications are highly accuratewith minimized false detections.

In the prior art, ion species have been identified by selecting Vmax andVc and detecting species passing through the filter field, or selectingVmax and scanning Vc to obtain a mobility scan of the sample. Compoundscan be identified according to this process. However, species whosespectra overlap can defy accurate identification. Peak shiftingtechniques of the invention enable separation of such hidden oroverlapping peak information.

For a given ion species in a sample, as the amplitude (Vmax & Vmin) ofthe asymmetric RF voltage changes, the amplitude of the DC compensationvoltage (Vc) required for passage of that species through the filterfield will also change. The amount of change depends upon the speciesinvolved. However, there is still the problem of separation andidentification of several overlapping spectra. Furthermore, making afirst detection and changing one field parameter alone is inadequate forimproving species discrimination, as such single change would retune thefield for detection of a different species rather than improvingseparation between detected species and being able to isolate andredetect the same species.

Thus, in practice of the present invention, an ion species is identifiedby making a provisional detection and causing and observing predictedbehavioral changes of ion species under selected operating conditions.This process benefits from “adjusting the knobs” and creating and usingmultiple data points to support a species identification.

In an illustrative embodiment of the present invention we improveaccuracy of species identification by detecting and provisionallyidentifying at least one ion species, typically one out of several ionspecies with overlapping spectra, at a first set of filter operatingconditions. We then selectively change these operating conditions basedupon knowledge of the first detection and upon predicting the affectsuch change will have upon the behavior of such provisionally identifiedspecies. Then we confirm the provisional species identification bydetection of the predicted species behavior. With this plurality ofpurposefully related data points we access our stored detection data andmake a species identification. (The techniques of data storage andaccess are well-known.)

In a special embodiment of the invention, a device is dedicated todetection of a prescribed analyte and detection is made at a prescribedset of operating conditions. Then positive and negative detection modedata, and/or data from detection at a second set of prescribed operatingconditions, is used for species identification according to theinvention.

Thus it will be appreciated that in a preferred practice of the presentinvention we improve species identification by improving separationbetween analyte peaks. We do this by controlling or manipulating aspectsof the filter operating space. We decide which parameters to adjustbased on known species behavior. The spectrometer can scan a spectrumand once a species is detected and provisionally identified then asecond detection is made to verify such provisional identification. Wecan also set the spectrometer to detect a given species or class ofspecies in this multi-step process of the invention.

In an illustrative practice of the invention, we note detection peakcharacteristics (such as polarity, peak, location, intensity, width,etc.) at a first set of operating conditions (noting ionization source,dopant level if any, and other electric and environmental fieldparameters, including Vmax, Vmin, Vc, RF frequency, and duty cycle). Wethen change at least one mobility-affecting parameter in the operatingconditions (such as adjusting dopant level, Vmax, Vmin, Vc, RFfrequency, duty cycle, etc.) and note changes in peak characteristics(such as location, intensity, width, etc.) at the second set ofoperating conditions. This collected data is compared to a lookup tableof detection data for known compounds in such conditions. Upon datamatch, a species identification is made with a high degree ofreliability. In the preferred practice of the invention, we make thesecond detection at a second set of operating conditions that isselected according to knowledge of the first set of operating conditionsand the first detection results.

As will now be understood, we selectively make adjustments to thefilter. These adjustments are made in terms of waveshape, Vmax, theratio of Vmax/Vmin, DC compensation, frequency, duty cycle, and/oraspect of asymmetry, and in terms of other environment variables likepressure, flow rate, gas composition, moisture, ionization process,and/or presence and level of doping. The result is improved speciesseparation and improved species identification.

Waveform Adjustments—Background Spectra

In any sample, there may be several spectra generated, including thosebased on low level background impurities in background spectra),components of the carrier gas (oxygen, nitrogen, etc., generallyreferred to as RIP), and analyte spectra.

FIG. 4 and FIG. 5 show a comparison of effect upon detected backgroundspectra, sometimes referred to as RIP, upon switching between two RFfrequencies 0.5 and 1.1 MHZ. The effect of variation in frequency onbackground spectra shows that for lower frequency (0.5 MHZ) the RIPpeaks for both positive (FIG. 4) and negative (FIG. 5) ions are locatedon the higher absolute values of compensation voltage (|−Vc|). Thisdemonstrates that changes in frequency correlate with changes in RIP,which may be applied for example when attempting to separate peakslocated near the RIP. This also demonstrates that without changing otherparameters, a characteristic spectra shift (ΔVc) can be attributed tothe RIP when switching frequencies. This can be used to confirmdetection of the RIP peak(s).

Thus in one illustrative method of the invention, such as shown in FIG.4, a scan is performed with RF at a first frequency, such as at 1.1 MHZat a Vmax such as at 660V. A positive mode peak is detected at a firstVc (such as at −4.0). We then redetect the peak at a second frequency,such as at 0.5 MHZ, and note peak location, such as at a second Vc ofabout −4.5, also in the positive mode. The change of Vc, between thefirst and second Vc indicate shift of an RIP peak, since analyte ionswill respond differently. This detection process identifies the RIP andwill enable separation of background spectra from analyte spectra duringanalysis of a chemical sample.

It will be appreciated that this detected data must be correlated withstored data for identification of the detected species. In this example,the background spectra can be identified based on stored datarepresentative of background spectra in that device and can now beseparated from analyte data to be collected and identified in thatdevice.

In the identification process of the invention, use of positive modedata of FIG. 4 can be augmented with use of negative mode data (FIG. 5shows negative mode data, i.e., data representing detection of negativeion species for samples of FIG. 4). These data can be detectedsimultaneously in practice of the apparatus of the invention, however,they can also be performed in sequence. In this case, the data set ofFIG. 4 was gathered in a field having a Vmax of 660 v, while in thesecond data set, FIG. 5, the background spectra was detected in a fieldset with a Vmax of 520 v.

Choice of multiple Vmax values illustrates that more than one variablecan be adjusted in practice of the invention, for example, with changesin both frequency and in field values. This follows because we predict asecond data point based on first provisional identification withoutrequiring linearity or other limits on changing parameters as long as asecond definitive set of data (e.g., characteristic peak shift) can begenerated to verify the first species detection.

Turning to FIG. 6A & FIG. 6B one can see that there are no differencesin peak position at the low RF voltages (low field strength) for the RIPfor the two frequencies shown. Peak shift due to frequency change isdiscernable at the voltages higher than about 500V, evidencing higherfield strengths.

Peak intensity is sensitive to frequency change. In FIG. 6C and FIG. 6Done can see that there are differences in peak intensity even at low RFvoltages. This likely results from having a small gap between the filterelectrodes (e.g., gap width of 0.5 mm in one practice of the invention).With a small gap, with decreasing frequency and increasing excursiontime, more ions can reach the channel walls and be neutralized. Thiseffect increases as the RF increases. According to FIG. 6D, in theseconditions the intensity of lighter negative RIP ions (mostly oxygen)decreased more, faster and disappeared at 600V for the lower frequencyand 650V for the higher 1 MHz. The heavier positive ions (H₂O/nH⁺) cansurvive at higher voltages, e.g., 650V (FIG. 6C), but again peakintensity was less for the lower frequency than for the higher 1 MHz.

These findings are important as they can be applied to improveddetection of analyte ions in a chemical sample, by further enablingknowledgeable separation of RIP and background spectra from analytespectra according to responses to applied changes. Collection of datafor background and analyte spectra enables creation of a table of datafor use in species identification.

FIG. 7A shows the effect of changes in frequency on the positivedetection peak position for acetone at two frequencies 0.5 and 1.1 MHZfor a field having a Vmax of 555 v. Note that as the frequency decreasesso does the required compensation. FIG. 7B shows the effect of changesin electric field strength (Vmax of 555 v and Vmax of 655 v) for a DMSfilter operated at 1.1 MHZ for acetone positive mode peaks in practiceof the invention. Note that as the RF field increases so does therequired compensation. It will be appreciated that these arecharacteristic detection data which can be stored for later use inidentification of detected ion species in the apparatus of theinvention.

It will be further appreciated that ionization of acetone not onlyproduces positive ions but also releases free electrons. These freeelectrons are expected to be captured by species with high electronaffinity. FIG. 8A shows the effect of changes in frequency on thenegative detection peak position for negative spectra having receivedfree electrons from ionization of acetone at two frequencies, 0.5 and1.1 MHZ, for an RF field having a Vmax of 555 v. FIG. 8B shows theeffect of changes in electric field (Vmax of 555 v and Vmax of 655 v) at1.1 MHZ for the negative spectra. Comparing peaks teaches that, for thehigher field, the required compensation is increased significantly. Thisincreased peak separation, measured as changes in Vc, significantlyassists in species identification in practice of the invention.

Thus it is clear that controlled changes in frequency generatecontrolled and predicted changes in known analyte behavior and can beapplied in a species separation and identification process of theinvention. In the case of any ionized analyte, these known responses inmobility behavior, such as changes in field strength or frequency, arethe basis for assembling stored data which is then used in identifyingdetection spectra. Additional stored data can be assembled foradditional characteristics and additional analytes.

It will be appreciated that various analytes can be characterized inthis manner. For example, FIG. 9A shows the effect of changes infrequency on the positive detection peaks for benzene at twofrequencies, 0.5 and 1.1 MHZ, in an RF field having a Vmax of 555 v;note that the lower frequency peak now has revealed additionalinformation as a second peak. In FIG. 9B, effect in changes in fieldstrength shows that the higher field at 555 v reveals a second peak asagainst the single peak of the lower field at 455 v.

FIG. 10A shows the effect of changes in frequency on spectra fornegative species related to ionization of benzene (i.e., species whichhave received free electrons released from ionization of benzene) at twofrequencies 0.5 and 1.1 MHZ for an RF field having a Vmax of 455 v. Asubstantial peak is detected at the higher frequency while the peak isattenuated at the lower frequency at higher compensation Vc. Thisadditional compensation represents increased peak shift. Knowing thiseffect, peak shift, when properly induced, enables separation of speciesand improved species identification.

FIG. 10B shows the effect of changes in electric field strength uponnegative detection peaks related to ionization of benzene (i.e., specieswhich have received free electrons released from ionization of benzene)in RF fields having Vmax of 455 v and 555 v at 1.1 MHZ. There is a firstpeak at 455 v and a lower peak shifted in compensation at 555 v.

As a further example of peak shift, FIG. 11A shows the effect of changesin frequency on the positive detection peaks for toluene at twofrequencies, 0.5 and 1.1 MHZ, in an RF field at Vmax of 555 v; note thatthe lower frequency peak is of lower intensity and is split into doublepeaks while the higher frequency peak has a more intense double and asmall third peak at different compensations. FIG. 11B shows the effectof changes in electric field strength between Vmax at 555 v and at 655v) at 1.1 MHZ for toluene positive mode peaks in practice of theinvention.

FIG. 12A shows the effect of changes in frequency on negative speciesrelated to ionization of toluene at two frequencies 0.5 and 1.1 MHZ foran RF having a Vmax of 455 v. A substantial peak is detected at thehigher frequency while the peak is attenuated at the lower frequency athigher compensation. FIG. 12B shows the effect of changes in electricfield upon negative detection peak position at Vmax at 455 v and 555 vfor an RF field at 1.1 MHZ. There is a substantial shift in peaks withthe lower field peak having a higher compensation.

Once, again, it will be appreciated by a person skilled in the art thatcontrolled changes generate controlled and predicted changes in analytebehavior and can be applied to replicate such behavior in a speciesseparation and identification process of the invention. Thus simple andcomplex samples can be analyzed in practice of the invention. Thesetools enable manipulation of ion species to improve species analysis(separation, detection and identification). This is based on the factthat the level of change in V_(c) and peak shape or intensity differ fordifferent ion species for different field conditions and changes.

In practice of the invention, parameters of waveform characteristics,such as frequency, can be adjusted for discrimination of ion species. Wecan use the effect, such as varying RF frequency, as an alternative toor in combination with varying of field strength. Variation of frequencyfor a given RF intensity can also enable additional species separationaccording to mobility, weight, mass or structure. For example, in highfrequency conditions the ion filter can pass a range of species, withgood separation between heavier and lighter ions. In low frequencyconditions only heavy ions will pass (lighter ions having enough time toneutralize on the electrodes will not pass). At low frequency, theseheavier ions will be better resolved in comparison with high frequencyconditions. For heavier ions, we use a high RF voltage and lowfrequency, in one practice of the invention, for improved speciesdiscrimination.

We can choose to vary frequency or other field parameters in generatingspecies data. In the simplest case, we can adjust the field strengthsince this may affect ion species behavior. However, this change aloneis not always adequate as a process control. Furthermore, a bettermeasure of species dependence is the ratio of Vmax to Vmin for theparticular RF field correlated with a given species behavior. Thismeasure brings in attributes of the waveform asymmetry and its impact onion species behavior in the filter.

In practice of the invention, the effect of RF field on the iontrajectory may be compensated with a variety of techniques. This mayincludes DC compensation or it may be provided by varying other aspectsof the filter operating conditions, the effect of which is to perform acompensation function. An example of this includes adjustment of aparameter of the waveform, such as duty cycle.

Different waveform shapes, such as different square waves will have animpact on species detection. A different waveform will elicit differentmobility behavior for some species, as evaluated by the level of andchanges in compensation. These are signature events that are noted andutilized in practice of the invention. An illustration is shown in FIG.13 and FIG. 14, regarding use of two different wave shapes (i.e.,flyback and square). In FIG. 13A a first waveform is shown, generatedwith a flyback generator, and is correlated with the spectra shown inFIG. 14A. The positive peak is fairly broad and represents backgroundand analyte. In FIG. 13B, a square wave is shown which correlates withFIG. 14B, showing resolution of peak P into peaks P1, the unresolvedanalyte of FIG. 14A, and the background spectra peak P2. Thisdiscrimination is achieved even where other field conditions remain thesame, showing that changes in waveform correlate with differentialchanges in compensation for different analytes in a sample. Thisdifferential behavior can be favorably used in practice of the inventionto improve species discrimination and identification.

While wave shape changes can be implemented as the above exampleteaches, other field changes can be imposed to increase species peakseparation. In FIG. 15A we show a combination of positive and negativemode detections of a sample containing toluene dopant and SF6, and theaffect upon the RIP, for detections at a fixed duty cycle of 0.2. Twodetections are made, one at 463 Khz and the other at 1 Mhz. In thepositive mode, the toluene and RIP are clearly discerned at bothfrequencies. SF6 follows the same pattern but in the negative mode. Itis noted that in the positive mode, at the lower frequency, there isbetter separation between toluene and RIP peaks even though at lowerintensity. It will thus be appreciated that this separation would be thetype of ion species behavior response sought to be achieved in thesecond step of the presently disclosed process of the invention.

Referring to the data of FIG. 15B and FIG. 15C, we demonstrate theuseful affect that adjustment to duty cycle has on species analysis inpractice of the invention. In FIG. 15B we show the effect of changingduty cycle of an RF Vmax at 692 at 463 Khz, for the negative spectra ofSF6. Detections were made at six different duty cycles from low of 0.09up to 0.216, showing a leftward shift (about 2 v) of the detectionpeaks. This is represented as stored data which then can be accessed toidentify the SF6 spectra in an identification process of the invention.

In FIG. 15C we show effect of changes in duty cycle of an RF having Vmaxat 692 at 463 Khz upon RIP and toluene peaks. Detections were made atsix different duty cycles from low of 0.09 up to 0.216. The RIP andtoluene peaks appeared to strongly overlap at the lower duty cycle butwere well resolved at the higher duty cycle, even as the detectionintensity decreased. Thus, again, practice of the invention suggeststhat analytical optimization can be counter-intuitive in that normallyefforts are made to maximize detection intensity. Yet we have shown thatmore useful detection data is obtained even at a loss of signalintensity because it is used in the second step of the disclosed processto confirm the first data, rather than as absolute data on its own whereintensity might be more critical.

Use of Dopant

We broadly define doping as the process of adding an analyte for thepurpose of affecting ion species behavior. We use doping to assist inidentifying analytes of interest. We define several forms of doping.

Doping may include the step of addition of an analyte in the ionizationprocess whose ionization releases free electrons which enablesionization of negative species. Doping may include the step of use of anadditive to improve ionization efficiency. Doping may include the stepof addition of an analyte that affects species behavior and causes peakshift. We use these functions in practice of embodiments of theinvention.

Ionization may be implemented through a variety of techniques, e.g., useof a radioactive source like ⁶³Ni, an ultraviolet lamp, a plasma orcorona discharge device, etc. Generally speaking, for successfulionization, the applied ionization energy must be at least as much asthe energy of ionization for the molecule of interest. For example, ahigh source of energy is required (such as ⁶³Ni) for direct ionizationof molecules having high energy of ionization (such as SF6). However, inmany circumstances it may not be possible to use a radioactive source toeffect such high energy direct ionization.

In practice of one embodiment of the invention, we use a non-radioactiveionization source 16 (e.g., UV lamp) where the energy of ionization isless than the energy needed for direct ionization of compounds such asSF6. We introduce a dopant into the ionization path (e.g., into theinfluence of photo-ionization from a UV lamp) in the ionization region14. In this arrangement, adequate energy is supplied to ionize alow-energy-of-ionization dopant (e.g., acetone, toluene or any substancewith energy of ionization less than energy of photons from the photonsource), which generates positive dopant ions and free electrons.

The dopant ions and free electrons are mixed with sample molecules.Molecules having a high electron affinity will be ionized by these freeelectrons. Thus molecules which normally cannot be ionized in UV can beionized in practice of the invention. The resulting ions are thencarried into filter 24 and detector 32 for detection and identification.

In a preferred practice of the invention, we introduce an adequate flowof dopant into the ionization region, at least enough that results in alarge volume of doping ions filling the volume. This increaseslikelihood of ionization of the analyte molecules by charge transfer.Therefore this use of dopant enables ionization and detection of traceamounts of analyte in situations where otherwise they are likely to bemissed, which results in increased detection sensitivity. In practice ofthe invention, the analyte ion peaks are detected and distinguished fromdopant peaks.

In one illustration, we use a dopant to improve ionization of SF6.Samples of SF6 were introduced along with a constant level of dopant(acetone) for UV ionization. The system was operated with RF voltage at1130 v, with dry air (humidity at 10 ppm), at atmospheric pressure. FIG.16A shows the negative mode response for different levels of SF6concentration. It is clear that this detection mode is consistent(without peak shift) for varying levels of negative SF6 ions. FIG. 16Bshows the positive mode response for the dopant used in this experiment,where, even with different levels of SF6, the positive spectra does notchange. This is a direct reflection of detection of positive ions of thedopant and negative ions of the analyte (SF6).

This experiment demonstrates the power of using a low energy ofionization dopant (e.g., acetone) to ionize a high energy of ionizationmolecule (e.g., SF6) without requiring use of a high energy ionizationsource. Therefore we can use a non-radioactive ionization source (e.g.,UV). We also benefit from the ability of using a detector (electrodes28, 30 of FIG. 2A) having modes which may simultaneously distinguishbetween and detect both the analyte and dopant ions.

Single mode detection can be adequate for identification of ion species,such as SF6. Meanwhile, in the positive ion mode, there is no easilydiscernible SF6 peak as against the background spectra. But the absenceof discernable detection in one mode has significance for SF6identification. In other words, no other species has been detected. Thuspositive and negative mode data may be collected in a scan,simultaneously, and the combination of presence and/or absence ofvarious datum may be combined for a dual-mode analysis of the sample andidentification of a detected chemical species, such as SF6, based onlookup functions (guided by control unit 40), according to an embodimentof the invention.

We also use dopant to increase efficiency of ionization. In the exampleif FIG. 18, benzene dopant at 2 ppm was employed for UV ionization ofheptanone. Spectra are shown for four scans, showing: background aloneand with dopant and heptanone without and with dopant. Differences inpeak detection and intensity are clear. The benefit of detectingheptanone with doping relative to detection of heptanone alone isdemonstrated as increased detection signal. This results in increasedsensitivity and selectivity in practice of the invention.

FIG. 19 illustrates use of benzene dopant at 2 ppm for UV ionization ofbutanone. As seen in FIG. 19, spectra are compared for background aloneand with dopant, and for undoped butanone and with dopant. Differencesin peak detection and intensity are clear. The benefit of detectingbutanone with doping relative to detection of butanone alone isdemonstrated as increased detection signal. This results in increasedsensitivity and selectivity in practice of the invention.

FIG. 20 illustrates use of benzene dopant at 2 ppm for UV ionization ofDMMP. As seen in FIG. 20, spectra are compared for background alone andwith dopant, and for undoped DMMP and with dopant. Differences in peakdetection and intensity are clear. In this case, the benefit of usingdoping relative to detection of DMMP alone is demonstrated. In FIG. 20,the scan of DMMP sample with benzene dopant produces three peaks “a”,“b”, “c”. Peak a relates to detection of background spectra, while peaksb and c relate directly to detection of DMMP. Peak “d” is a minor peakfor DMMP detected without doping.

The three peaks a, b, c for doped DMMP is a signature constellationrelated to DMMP. If detected under these conditions it can be compareagainst stored data for positive DMMP identification. Note that thisconstellation of peaks and their locations, is different for thesignature for doped heptanone and butanone in FIGS. 18, 19. Such storeddata can be accessed accordingly for species identification.

But the DMMP can be identified against its own data, regardless ofcomparison to spectra for other analytes. For example, the small peak dfor DMMP without dopant might be confused with detection of severalsimilar analytes (e.g., butanone and heptanone). However, once that peakis detected, a dopant can be supplied (e.g., benzene at 2 ppm) andresulting spectra can be obtained (e.g., spectra a, b, c FIG. 20)enabling accurate identification of DMMP with a high degree ofconfidence.

Thus in a multi step process of the invention, results of a detectionwithout doping suggest system changes for a second detection. Forexample, detecting peak d suggests a group or class of analytes (in thisexample butanone, heptanone and DMMP). Yet if followed by the dopingshown above, analyte peaks are definitively separated enabling specificidentification of analytes in the sample.

It will be appreciated that we broadly define doping as the process ofadding an analyte for the purpose of affecting ion species behavior. Theforegoing demonstrates use of doping to generate negative species, suchas SF6, and use of doping to improve ionization efficiency and detectionsensitivity.

We also use the term doping to include the step of addition of dopingthat affects species behavior in such a manner as to change thecompensation required to pass the species through the filter. Thisresults in a shift of detection peak(s), usually measured in a changesin DC compensation voltage. This is similar to use of electric fieldchanges to shift peaks, described above. Here, again, creation of alookup table of data reflecting the affect of a given dopant on peakshift for a given set of filter conditions for a given analyte enablesimproved separation, detection and identification of analyte.

We can use various polar molecules as dopants in practice of theinvention. FIG. 21 shows relative performance of four chemicals (MC,propanol, acetone and water) as dopants for use in detection of theexplosive DNT, measuring concentration versus amount of compensation inpractice of the invention. The best performance for detection of DNT wasobtained from MC (methylenechloride or dichloromethane), which provideda greater shift in compensation per unit measure. This experienceextrapolates to detection of other explosives, including NG, NS, NC,DNB, PETN, TNT, among others. Several peak shifting doping examplesfollow, however the invention is not limited to use of MC or to any oneexample.

In FIG. 22A-E we show detection spectra a-e for explosive agents NG,DNB, DNT, TNT, and PETN, as elutes from a GC, comparing retention timeto compensation voltage, without dopant. In FIG. 23A-E we show betterdefined and separated detection spectra a-e for the same agents using MCdopant. This benefit is most clearly seen in FIG. 24A-B. ((FIG 24A is acomposite view of spectra without doping and FIG. 24B is a compositeview of spectra with MC doping. In practice of an embodiment of theinvention, we calculate the amount of peak shift for each analyte causedby this doping and store this information as identification data forlater use in a lookup table for explosives identification.

While use of MC as a dopant for explosive detection and peak shifting isnew, we have also found that use of MC simultaneously suppressesbackground spectra. The result is improved detection sensitivity,capability and efficiency. Still additionally, we are able to use thissame MC dopant gas for purging of the DMS system in a possibleadditional step of the invention. Use of MC, as one of several favoreddopants, is therefore advantageous in practice of the invention.

In practice of a multi-step embodiment of the invention, we make a firstdetection without dopant and a second detection with dopant. We improveidentification of species by use of doping-induced peak shifts asgenerating characteristic identifying data. FIG. 25A-E shows DMS spectrafor each of the same explosives: alone, MC alone, and as doped with MC.FIG. 25A shows NG having an undoped peak al at ˜0 v and a significantshift in compensation for the doped peak a2 at ˜13 v.

FIG. 25B shows DNB having an undoped peak b1 at ˜2 v and a significantshift in compensation for the characteristic DNB doped peak b2 at ˜21 v.FIG. 25C shows DNT having an undoped peak c1 at ˜2 v and a significantshift in compensation for the characteristic DNT doped peak c2 at ˜19 v.FIG. 25D shows TNT having an undoped peak d1 at ˜0 v and a significantshift in compensation for the characteristic TNT doped peak d2 at ˜10 v.FIG. 25E shows PETN having an undoped peak e1 at ˜0 v and a significantshift in compensation for the characteristic PETN doped peak e2 at ˜8 v.

These analyte-related peak shifts are signatures which can be used toidentify detected species. It will be further appreciated thatadditional information may be obtained and used in this process. Forexample, shift of the MC-related peak adds additional characteristicinformation. In FIG. 25A the MC-related peak a3 is at ˜0 v and is partof the signature of the NG+MC cluster peak a2, while the combination ofboth makes for an accurate signature of NG in this example. This is truefor the other analytes, however in FIG. 25B-C the MC peak is not shownbecause it is off-scale.

These figures demonstrate that use of dopant and amount of dopant arecontrols that can be used to obtain peak shifts according to theinvention. Changes in peak position (which may be measured in terms ofcompensation voltage) can be used as part of the identification practiceof the invention. In one embodiment, we provisionally identify ananalyte, add dopant to change the filter conditions to adjust ormanipulate the peak position, predict the analyte peak shift, confirmpredicted behavior, and therefore make a confirmed analyteidentification. This enables a highly reliable analyte identificationprocess with a high degree of confidence in practice of the invention.

As shown in FIG. 21, we can use various polar molecules as dopants inpractice of the invention. In one practice of the invention, we controlof humidity in the filter environment to provide species separation. Itis noted that atmospheric pressure chemical ionization processes areknown to be affected by moisture. However, quite unexpectedly, we havefound that once the ions are passed into the analytical region of theDMS systems of the invention, unlike conventional ion-based systems,higher levels of moisture actually increase resolution rather thandegrade it.

In general there is minimal effect of moisture below 100 ppm on the DMSspectra. This is consistent with IMS where only above 100 ppm does onestart seeing shifting of peaks and loss of resolution. There are severalpossible approaches to controlling the effect of moisture in the DMS.One is by physical means, through controlled addition or removal ofmoisture (membranes, permeation tubes, temperature). Another means isthrough the use of algorithms. As an illustration it is possible totrack the RIP peak position as a humidity indicator.

We can apply the control process of the invention to various polarmolecules, such as water, in detection of analytes, includingexplosives, chemical warfare agents, and the like. FIG. 26A-C showsspectra for different concentrations of water in air, for DNT peaksshifted in different concentrations of water, and we plot the relationof DNT peak position to water concentration. FIG. 26A shows how thereactant ion peak position moves away from zero to higher compensationvoltages with increasing moisture levels. FIGS. 26B and 26C show thechemical peak shifting in response to increased moisture levels.Generally the DMS spectra with moisture levels from 50-10 ppm are verysimilar. Notice, in FIG. 26A there is only a slight shift in the RIPpeak position from 60-150 ppm.

In FIG. 26A the peaks are distributed from about 11 v to 30 vcompensation. In FIG. 26B we add DNT to the sample and analyze the sameseveral concentrations of water. The same distribution of water peaks isagain shown, but now we also see DNT/water peaks at various compensationvoltages. It will therefore be understood that depending upon the levelof water dopant, we can cause a definable or predictable shift in theDNT peak. For example, we can use water at 150 ppm and detect the DNTpeak at around 5 v and then we can switch to water at 600 ppm and detectthe DNT-water cluster peak with a shift of around 3 v. This shift ischaracteristic to DNT/water and thus we use detection of an expectedshift to confirm presence of DNT using water as a dopant control knob inone practice of the invention.

The present invention is not limited to detection of any particularclass of analyte. The following examples demonstrate improved detectionand identification of organophosphorous compounds using waterconcentration to shift peaks. FIG. 27A-C shows one example of theeffects of moisture for the DMS separation of gas phase ions. Atime-varying filed between 0 and 25 kV/cm was applied at ambientpressure for protonated monomers [(MH+(H2O)n] and proton bound dimers[M2H+(H2O)n] of organophosphorous compounds. As shown in

Turning to FIG. 27A we show detection of DMMP, DEMP and DIMP. Theseanalytes have similar properties and overlapping spectra, especially theDIMP and DEMP, at very low moisture, however they shift substantially athigh humidity. Thus the cluster of chemicals can be provisionallyidentified at low humidity. Then a second detection is performed at highhumidity and the shift of peaks is observed. FIG. 27B showscharacteristic response for DMMP, DEMP and DIMP at 95000 ppm. Comparingthe shift data between FIG. 27A and B, it will be noted that DIMP shiftsfrom about −2.5 to about −10.5, DEMP shifts from about −3 v to about −13v, and DMMP shifts from about −6 v to about −30 v. This shift providesimproved peak separation between the analytes. FIG. 27C shows peakposition versus moisture for the three analytes.

Thus it will now be understood that in a salient aspect of the inventionwe can provisionally identify an analyte, change the filter conditionsbased on predicted behavior of that analyte so as to adjust ormanipulate its peak position, we confirm the predicted behavior, andtherefore we verify analyte identification. This enables a highlyreliable analyte identification process with a high degree ofreliability and reduced false positives.

Returning again to the embodiment of FIG. 2C, a multi-channel system 11is shown including dual flow paths 13 a, 13 b. Having a plurality ofchannels enables running identical processes in the channels ordifferent processes in each channel. In the latter case variations inelectrical (waveform, etc.) or environmental (pressure, humidity, etc.)conditions in the flow path can be used to improve species detection andidentification. This plurality of flow paths enables collection ofmultiple detection data for a sample or samples, which enables improveddetection analysis and more reliable species identification. Thus itwill be appreciated that the detection results of both flow paths 13 a,13 b can be used additively, subtractive, comparatively or otherwise todifferentiate, isolate and/or identify detected chemical species,raising confidence in species identification.

In one practice of system 11, an ionization dopant (A-dopant) andchemical sample are introduced at inlet 12 a and pass through ionizationpart 14 a into flow path 13 a. The ionized A-dopant enhances sensitivityof the system by increasing efficiency of compound ionization, asearlier discussed. (An alternative location of input port 12 is shown at12 a′.)

The ionized sample flows in the carrier gas/dopant toward and isfiltered at filter 24 a in one embodiment, for downstream detection,including simultaneous detection of positive and negative ions atelectrodes 28 a, 30 a of detector 32 a.

In a further practice of system 11, either positive or negative ionsfrom the ionized sample flow are directed into flow path 13 b viaorifice 25 and by action of properly biased steering electrodes 25 a, 25b. Now the ions in flow path 13 b are carried by a transport gas frominlet 12 b into ion filter 24 b, and are filtered and detecteddownstream in detector 32 b, accordingly. This plurality of detectiondata from detectors 32 a, 32 b, provides for improved speciesidentification.

In a further embodiment of the invention, the ions entering into flowpath 13 b are subjected to a resolution dopant (B dopant) that isincluded as or in the transport gas introduced at inlet 12 b. TheB-dopant improves peak resolution by differentially impacting spectralpeak position (i.e., characteristically effecting the amount ofcompensation voltage), which will depend upon ion-mobilitycharacteristics of the detected analyte(s).

As will now be appreciated, these and other embodiments of themulti-channel system 11 enable control or manipulation of the analyticalfunction within one or several flow path(s) for obtaining improvedspecies separation and identification.

Regulation of Pressure

In conventional DMS spectrometers, the ions from the sample are carriedby a carrier gas through the system. In conventional IMS systems acounter-flowing gas stream is used essentially for cleaning the driftregion. In any event, to the extent that the gas is intermingled withthe sample ions, the presence of the gas in the ion separation regioncan complicate ion detection due to gas phase interactions or reactions,significant diffusion processes, and formation of dimer bond complexes,and so forth. This reduces the sensitivity and resolution of the system.

In these prior art practices, the presence of a high density gas mixedin the ion population requires the use of a large electric field toeffect ion discrimination. As a result, the power consumption isincreased. Power consumption is a very critical parameter for portabledevices.

Looking now at FIG. 28, there is shown a novel ion mobility-basedspectrometer 100 where a carrier gas 102 and sample 106 (such as SF6)and, optionally dopant 104, are mixed in a chamber 108, and areintroduced into a filter system 110. Ion filter system 110 may include afield asymmetric ion mobility spectrometer, such as spectrometer 10described above, in which case the ion mobility spectrometer willinclude filter 24 and detector 32 discussed above; or system 110 mayinclude another type of ion mobility-based spectrometer, such as atime-of-flight ion mobility spectrometer. Flow rate is regulated by avalve 112. System pressure is controlled via pump 114 and valve 116.

In practice of the invention we have found that by regulating (andpreferably reducing) the pressure of the system, system sensitivity canbe improved. When we reduce the pressure, less carrier gas is presentamongst the target ions so that there is, among other things, less iondestruction (e.g., through quenching) and less masking of the mobilitycharacteristics of the ions (e.g., due to random collisions occurringwithin the filtering electronic field). In addition, as the gasconditions (density N or pressure P) are reduced, the electric field canalso be reduced (maintaining the E/N or E/P ratio), so that powerconsumption can be reduced. Hence system sensitivity can be improved andpower consumption can be reduced. This is particularly advantageous inmaking a hand-held detector.

FIG. 29 shows effect of changes in pressure on negative and positivebackground spectra, when all other parameters, flow rate, RF voltage,temperature, are constant. In this illustrative experiment RF was 1300V.The system was stable and test results were reproducible for same andother RF voltages and pressures.

Analysis of these spectra shows that with decreasing pressure theabsolute value of compensation voltage for both (the positive andnegative) RIP peaks is increased (peaks are shifted to left), whileimpurity (or cluster) peaks around zero compensation shift in theopposite direction. Also, peak intensity decreases with reducedpressure. The effect of pressure is stronger for negative peaks versuscompared to positive mode peaks, wherein the negative mode peak shift isgreater and intensity decreases more rapidly.

For lower pressure conditions, the peaks become broader, probably due toincreased separation between different species of ions. For example, itis understood that in the case of positive mode the RIP peak speciesinclude combinations of protonated water peaks (H2O)nH+, and in negativemode these species include combinations of oxygen-containing ions, suchas (H2O)n O2—. (The level of clustering (n) depends upon the level ofmoisture.)

The quantified effect of pressure on peak parameters (shown as peakintensity and compensation voltage in FIG. 29) may be directly observedin FIG. 30. This data shows that the level of effect on peak parametersis more significant at lower pressure conditions.

In FIG. 31, we show the level of influence of pressure on peak positionin terms of RF voltage. We have observed that the shift of RIP afterchanging pressure can be compensated by changing RF voltage so as toreturn the peak to the previous Vc indicating peak position. The changein pressure correlates with the new RF voltage.

We have observed that making a change in pressure has a different impacton positive and negative RIP peaks (compare FIG. 29 and FIG. 30).Therefore, adjusting positive and negative peaks requires differentlevels of RF voltage correction. FIG. 31 demonstrates adjusting thepositive mode peaks back to their initial position and noting the new RFvalue; the resulting negative peak position is offset from its originalposition. In general, this offset can provide information as to thedifference in the characteristic alpha parameter between the positiveand negative modes for this ion species. (See M070 for further alphadiscussion.) In general we have found that the level of effect ofpressure is increased with decreasing pressure.

FIG. 31 shows background spectra for positive and negative modes atdifferent pressures and RF levels, for a set flow rate. The key showsthe combination of RF voltage and pressure required to keep the positiveRIP peak at the same position compensation position (measured as Vc).From analysis of this data one can see that for lower pressureconditions a lower RF voltage was required. Again the analysis showsthat the level of pressure effect (i.e., the amount of required RFadjusted voltage) increases with decreasing pressure. For example, forchanging pressure P˜100 mmHg (between 760-655) the required adjustmentof RF voltage was V=1050−1010=40V. Meanwhile, for the same pressurechange at a lower pressure range (655-556 mmHg), the value of adjustedRF voltage was more than two times V=1100−920=90V.

In this experiment the peak intensity does not change as dramatically asit did when the electric field was not compensated (FIG. 29 and FIG.30). Explanation: the trajectory of recorded ions movement in theanalytical gap is not significantly changed; this follows becauseincreasing velocity of transfer direction movement (υ=K*E) due toincreasing coefficient of mobility at lower pressure conditions iscompensated by decreasing RF electric field. Non-monotonic behavior ofnegative peak position in this experiment can be explained in terms ofthe differences of a parameter of positive and negative ions species.

The quantified effect of electric field compensation for pressuredecreasing may be directly observed in FIG. 32. This data may be usefulfor practical applications. For example, at reduced pressure, the DMSwill have increased resolution compared to atmospheric pressure. Aswell, operation at lower pressure requires lower RF voltage andtherefore decreased power consumption and reduced sensor and drivecircuit design requirements. For example, according FIG. 32 one can seethat by decreasing pressure to 0.3 atm the RF voltage is decreased aboutin half.

FIG. 33 shows the effect of different pressures on negative (A) andpositive (B)spectra of TBM (tert-Butylmercaptan or tert-Butylthiol)[C4H9SH], with RF voltage held constant.

FIG. 33 shows that the TBM spectra also are changed according to changein pressure. Direction for the peak position changing is opposite toshift in RIP peaks. Level of change is less than for RIP (see FIG. 29).For example, the peak position and intensity do not change approachingatmospheric pressure (between 760-650 mmHg). The quantified effect ofchanging TBM peak parameters is shown in FIG. 34, showing the effect ofpressure on negative (a) and positive (b) TBM ions peaks parameters.

In another illustration, it was seen that RIP peaks are more sensitiveto changing pressure than TBM peaks. Direction of peak shifting for RIPpeaks and TBM was opposite. With decreasing pressure, RIP peaks shiftedin the direction of increasing absolute value of compensation voltage,while the TBM peaks moved in opposite direction. Thus it now will beunderstood that changes in pressure yield predictable changes to speciesand therefore can be pressure can be used as a “knob” which can beadjusted to separate detection peaks and improve identification ofcompounds in a sample.

It will therefore be appreciated that changes to pressure impactsbackground and analyte spectra. The present invention makes use of thequantifiable effect of pressure on peak parameters.

In one experiment, shown in FIG. 35A and FIG. 35B, SF6 at 250 ppm wasionized with acetone dopant in UV ionization, where spectrometer 10 wasoperated at an RF of 500 v, at 0.3 atmosphere and with laboratory air.In the negative mode (FIG. 35A), the background spectra without SF6 andthe detection peak for SF6 are shown. The SF6 detection peak appears ata compensation voltage of about −5.5 volts, while in the positive mode(FIG. 35B) the acetone dopant was detected at a compensation of about −9volts. Operating at 500 v is significant, since this is sizablereduction in RF voltage, thereby resulting in lower power consumptionwhile still providing excellent SF6 identification capability.

As well, in this example, the lab air was at about ˜5000 ppm humidity.Thus it will be appreciated that the present invention allows SF6 to beeasily ionized using a non-radioactive source with the assistance of adopant, and detected in low electric operating conditions (e.g., with anRF voltage of approximately 500 v) at reduced operating pressure (e.g.0.3 atm), even with elevated humidity (e.g., 5000 ppm).

It will be appreciated further that we have found that ionization of ahigh energy of ionization chemical, such as SF6, may be quenched in thepresence of high humidity and oxygen. Thus, the invention overcomesquenching by lowering the operating pressure of the apparatus; thisreduced pressure effectively decreases the effect of humidity andoxygen.

It is further noted that it is preferable that the ratio of electricfield to gas conditions, density N or pressure P, expressed as E/N orE/P, should be monitored and adjusted to obtain uniform detectionresults for a given compound. Practice of embodiments of the presentinvention enable reducing gas operating pressure which not only resultsin better ionization, such as for SF6, but it also allows the electricfield to be lowered while maintaining the E/P ratio. Thus, a reductionin operating pressure reduces power consumption, thereby permitting asmaller, lighter-weight, lower-cost and lower-power device.

Flow Control Apparatus

It is known that ion mobility is affected by factors such as particlemass, particle charge and particle cross-section. As well, control ofpolar molecules (such as H2O, CO2, NO2, NH4, etc.) can be used tofavorably affect mobility and detection. More specifically, we havefound that by adjusting or optimizing the humidity, and/or theconcentration of other polar molecules, in the sample, we can improvedetection sensitivity in practice of the invention. This is surprisinglytrue, notwithstanding our example above describing low pressuredetection of SF6 which seems insensitive to high moisture level.,

In practice of the invention, a sample can be adjusted by removing oradding appropriate molecules before or after ionization. In some cases,depending upon the sample, a reduced level (such as reduced humidity)can reduce clustering and can improve system sensitivity. This is truewhere clustering changes mobility and therefore masks the identify of acompound of interest.

But alternatively, in some circumstances, it may be advantageous tointroduce polar molecules into the sample to encourage clustering. Forexample, where it is difficult to otherwise differentiate between twodifferent ion species, addition of selected polar molecules can enablethese ion species to be separated, if they have different clusteringcharacteristics. Thus removal or addition of polar molecules can be usedas an additional control in the detection process in practice of theinvention.

In one illustrative embodiment of the invention, shown in FIG. 36,ion-mobility based spectrometer 200 of an ion detection andidentification system 201 is provided for analyzing a sample 205. Thesample is first passed through a humidity adjustment region 210 prior tobeing introduced into spectrometer 200. In a broadest aspect of theinvention, humidity adjustment region 210 may be any apparatus adaptedto adjust the humidity of the sample prior to introduction intospectrometer 200, e.g., by reducing or adding humidity to the sampleprior to or after ionization. In practice of an embodiment of theinvention, system 201 may includes a DMS spectrometer such asspectrometer 10 described above or may comprise a time-of-flight IMSsystem, or the like as spectrometer 200.

Turning to FIG. 37, there is shown a system 201 of the invention forcontrolling the humidity of a sample introduced into ion mobility-basedspectrometer 200. For example, in the case of reduction of humidity, aninput line 215 carries the sample from a source to the input 220 of ionmobility spectrometer 200. An output line 225, connected to the output230 of spectrometer 200, receives the spectrometer exhaust and carriesit off. Input line 215 extends through at least a portion of theinterior of the output line 225, and is formed at least partly out of awater-permeable membrane 235, such that water contained in the sampleflowing through input line 215 may pass through the wall of input line215 and be carried away by output line 225. In this respect it will beappreciated that inasmuch as output line 225 is typically connected to apump which purges the contents of output line 225, output line 225 willtend to have a lower pressure (P1) than input line 215 (P2), such thatP1<P2, so as to induce moisture to pass through the water-permeablemembrane wall of the input line 215 and into the interior of output line225 in the exhaust.

FIG. 38 illustrates an alternative approach for reducing the humidity ofa sample prior to introduction into ion mobility-based spectrometer 200.Here, system 201 includes input line 215 and output line 235 sharing acommon water-permeable wall 250 formed of a water-permeable membrane 255such that water contained in the sample flowing through input line 215may pass through wall 250 and into output line 225, where it will becarried away by the spectrometer exhaust. Again, a pump or otherarrangement may be employed to assure that the input pressure P2 isgreater than the output line pressure P1, so as to assist the moisturetransfer through the membrane for exhaust.

In several methods of the invention, we detect known species andcorrelate with RF field, compensation, pressure, humidity, and/or otherparameters. We create a data store describing at least one analytepreferably at various parameter levels. In one embodiment, the datasource is accessed as a lookup table.

Now we detect and identify a compound based on comparison to this storeddata. A single comparison may be adequate where a system is dedicated todetection of a particular species. An optimized set of RF andcompensation values may be selected along with values representingselected pressure and humidity. These optimized parameters are selectedto meet the criterion of increased reliability in identification by asingle detection set. Presence or absence of a species can be indicatedby conventional announcement means.

However, in another practice of the invention, we include the process ofdifferential peak shifting. This peak manipulation is based on ourobservation that different ion species of chemicals exhibit differentmobility behavior as a function of different operating conditions andthat as operating conditions are changed peaks will shiftcharacteristically. Thus we can develop a family of measurement datathat are characteristic of a given compound, including peak locationand/or shift data. We can record such data and use it for comparisonwith detection data when detections are made of unknown compounds inthose selected operating conditions.

We also have found one or several parameters of the filter conditionsthat can be selected and adjusted to achieve peak shifting after aspecies is detected and provisionally identified. Different speciesshift differently and characteristically. Upon detection of acharacteristic shift, the provisional species identification is verifiedand announcement of species identity is made with confidence. Thus, inone method, we can provisionally identify at least one peak even in thepresence of overlapping peaks, making a provisional speciesidentification, and based on the effect of known operating conditions.We manipulate these operating conditions and observe the effect of theadjusted operating conditions upon peaks. It is noted that the amount ofpeak shift is typically species specific and enables speciesidentification by amount of shift as one parameter. We correlate shiftsin peak position and intensity with operating conditions and referenceour stored data to make a species identification.

It will now be understood that it is possible to control operatingconditions and to discriminate between compounds that are ordinarilydifficult to separately identify by other means. Selection of operatingconditions enables isolation of an ion species of interest. Furthermore,because the system of the invention matches detection data with storeddata, we can select operating conditions that will produce detectiondata that is matchable to stored data, to determine a species is presentin the sample.

It should be furthermore understood that the invention is applicable notonly to field asymmetric ion mobility systems but may be applied ingeneral to ion mobility spectrometry devices of various types, includingvarious geometries, ionization arrangements, detector arrangements, andthe like, and brings new uses and improved results even as to structureswhich are all well known in the art. Furthermore, in practice of anembodiment of the invention, the output of the DMS filter may bedetected off board of the apparatus, such as in a mass spectrometer orother detector, and still remains within the spirit and scope of thepresent invention.

It will now be appreciated by a person skilled in the art that weoptimize ion species analysis in practice of embodiments the inventionby adjustment of operating conditions. These knobs are defined to enableadjustment of field, DC compensation, frequency, duty cycle, asymmetry,pressure, flow rate, gas composition, moisture, and/or ionizationtype/energy, among others.

Practices of the present invention may benefit from or be applied to asystem which incorporates the teachings of co-pending U.S. patentapplication Ser. No. 10/187464, filed Jun. 28, 2002, by Lawrence A.Kaufman et al., for SYSTEM FOR COLLECTION OF DATA AND IDENTIFICATION OFUNKNOWN ION SPECIES IN AN ELECTRIC FIELD (Attorney Docket No. M070),incorporated herein by reference.

The high sensitivity, rugged design and ease of use and setup of theinvention are advantageous for many applications that involve chemicaldetection. A simplified hand-held device of the invention is dedicatedto detection at just two “data points”, and yet reliably detects andidentified the ion species of interest. This practice may be augmentedby dual mode detections. The result is added reliability in chemicaldetection in a simplified device.

It will now be appreciated that in practice of the invention we optimizethe filter field, its electrical properties and its environment, in anion-mobility-based system to amplify differences in ion mobilitybehavior. Species are then separated, detected and identified based onthis optimization. We can further optimize the process by detecting ionpolarity, and we can optimize ionization and/or separation by usingdopants. Thus in practice of the present invention, we apply variousstrategies for improved isolation, detection and identification ofchemicals in a sample based on aspects of ion mobility behavior.

It should, of course, also be appreciated that numerous changes may bemade to the disclosed embodiments without departing from the scope ofthe present invention. While the foregoing examples refer to specificcompounds, this is intended to be by way of example and illustrationonly, and not by way of limitation. It should be appreciated by a personskilled in the art that other chemical molecules may be similarlyionized and detected, with or without the use of dopants, and/orpressure regulation, and/or humidity adjustment, and/or adjustment ofthe concentration of other polar molecules.

Therefore, while this invention has been particularly shown anddescribed with references to the above embodiments, it will beunderstood by those skilled in the art that various changes in form anddetails may be made therein without departing from the scope of theinvention encompassed by the appended claims

1. A DMS method for identifying chemical species in a sample, includingthe steps of: a) providing a DMS filter field, said filter field beingadjustable to a plurality of DMS filter operating conditions, said DMSfilter operating conditions being characterized as influencing mobilitybehavior of ion species in said field, b) processing an ionized samplein said filter field at a first set of said operating conditions, saidionized sample including at least one ion species, said processing beingbased upon aspects of mobility behavior of said at least one ion speciesin said first set of operating conditions, c) detecting a spectral peakassociated with said at least one ion species and said first set ofoperating conditions, d) provisionally identifying said at least one ionspecies based upon said detection and said association and by referenceto a store of detection data, e) changing said operating conditions ofsaid filter based upon said provisional identification and predictingthe effect of such change upon said provisionally identified at leastone ion species measured in terms of change in at least onecharacteristic of said spectral peak, f) detecting again said spectralpeak associated with said at least one ion species at said second set ofoperating conditions and confirming said predicted effect, g) based uponsaid first detection and said confirmation, verifying said provisionalidentification of said detected at least one ion species for identifyingsaid at least one ion species.